1. Field of the Invention
This invention relates to apparatus for grinding twist drills each having two clearance surfaces and a helical flute associated with each clearance surface, when apparatus comprises a rotatable drive shaft, a grinding wheel having a grinding surface and axially movably and tiltably mounted on and coupled to said drive shaft and spring biased toward a drill-guiding structure, which faces said grinding surface and has guiding passages differing in diameter and adapted to guide respective ones of said twist drills differing in diameter toward said grinding surface until the latter engages such twist drill at one clearance surfaces thereof, also comprising in alignment with each of said guiding passages at least one guide projection for extending into a flute of a twist drill extending through said guiding passage, and a drill stop means for engaging a twist drill extending through said guiding passage at the other clearance surface thereof so as to limit the advance of said twist drill toward the resiliently yielding grinding surface, said drill-guiding structure also comprising a plurality of abutments each having an abutment surface which is engageable by the grinding surface in an end position in which said grinding surface is closer to the guide projections than the drill stop means.
2. Description of the Prior Art
Such apparatus in which twist drills can be ground without being chucked are known from U.S. Pat. No. 4,574,528.
In a preferred embodiment of such known apparatus the axes of the guiding passages are parallel with and at an equal distance from the drive shaft and are meeting the inner rim of the angular internal conical ginding surface. That clearance surface of a drill being ground which does not engage the drill stop means will be forced against the grinding surface so that the spring-biased grinding wheel will be clear of those abutments which are closest to the twist drill but the grinding wheel will still be forced by the spring against the abutments which are diametrically opposite to the drill so that the grinding surface should be continuously restored to the desired shape during the grinding of a twist drill. It is expected that thereby no groove will be formed in the grinding surface during a prolonged grinding of twist drills which are so small in diameter that they will not contact the entire grinding surface during a revolution of the grinding wheel.
However, in such known apparatus which has abrasive grains of alumina in the grinding wheel and abutments of cemented carbides, the restoration of the original shape of the grinding surface will not always be completely maintained, especially if the difference in diameter between the smallest and the largest drill receivable in the apparatus will exceed a critical magnitude and if predominantly the smallest drills are ground. The reasons are that the abrasion of the wheel is essentially proportional to the pressure exerted onto its grinding surface. This pressure is highest in case of the smallest drill engaging the grinding surface. Hence, in order to prevent the formation of a groove in the grinding surface, the pressure between the abutment surfaces of the abutments and the grinding surface has to be at least as high as the pressure between the smallest drills and the grinding surface. To achieve this, the size of the abutment surfaces and hence their width would have to be so much reduced that abutments of cemented carbides would be worn considerably faster in the axial direction of the grinding wheel, which would considerably reduce the apparatus' durability.
In order to be able to explain a further disadvantage of the known apparatus, there has to be explained a disadvantage inherent in the grinding method under consideration.
The spring power, by which the grinding surface is urged against the clearance surface of the twist drill, is equal at all drill diameters. Consequently, the grinding pressure and hence of the grinding action in the direction of the axes of the drill (the axial grinding action) decreases with increasing drill diameter proportionally to the square of the drill diameter. Since the desired symmetrical point of the drill is achieved by grinding both clearance surfaces for an equally long time, time imperfections would cause relatively large deviations from a symmetrical drill point, when the axial grinding action is high. Consequently, the problem arises that at a spring power adapted for the smallest drills, the axial grinding action at the largest drills would be so low that an undesired long grinding procedure would be necessary for those largest drills.
Further, if in the known apparatus mainly small drills will be ground, there arises, as explained before, at least a slight groove in the corresponding area of the grinding surface, whereby the abutments loose contact with this area. In that area of the grinding surface which is not in contact with those small drills, the abutments not only wear out but also dull the grains in the grinding surface. Consequently, there arises an area of the grinding surface worn out by the smaller drills and having sharper grains, hence higher axial grinding action, and another area worn out by the abutments and having dulled grains, hence lower axial grinding action. However, because of the special geometry of twist drills it is the latter area in which the larger part of the material, which has to be ground away from these larger drills, is located. Consequently, this dulling effect of the abutments provides an additional and essential reduction of the axial grinding action just for those drills, namely the larger ones, for which already this grinding method leadsto a reduced axial grinding action. Indeed, in the U.S. Pat. No. 4,574,528 means reducing this dulling effect are provided. but these means increase the inclination for groove formation.